Deprecate the `\Doctrine\ORM\Query\Parser::setCustomOutputTreeWalker()` method #11641

…)` method We use this method only from within one of our own test cases, and I don't see how it would be useful to anybody else outside – it has to be called on the `Parser` instance which exists internally in the `Query` only. Deprecating and removing it in 3.x allows for a slight simplification in the `Parser` there, since we do no longer need the field (it can be a local variable).